They started with just one blog and moved on to other Washington Post properties:

We did a test with Ezra Klein’s blog by converting it to WordPress. He loved it and the blog has seen such a massive increase in pageviews we’re now moving all our blogs to WordPress and we’re launching new blogs every week.

and are big proponents of iteration and prototyping, which they did with their mobile site:

When we wanted to redesign our mobile site we started doing rapid prototypes with foundation and WordPress. Rather than building a Washington Post mobile site with articles, we wanted to build a mobile site with a great reading experience that happened to be by The Washington Post. This was obviously going to be a radical departure from most mobile news sites so to get buy in we had to have real prototypes management could use and approve.